A resistor-capacitor (RC) circuit, such as RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current signal source. A first order RC circuit can be composed of one resistor and one capacitor in series, where the product of resistance and capacitance is normally referred to as the time constant. RC circuits can be used to filter a signal by blocking certain frequencies and passing others. The three most common RC filters are the high-pass filters, low-pass filters, and band-pass filters.
An RC time constant is a value, measured in units of time, indicating the amount of time required to charge a capacitor from zero to approximately 63.2% of its full charge through a resistor. For various circuits, e.g. an integrated circuit, an RC time constant can be a very important parameter affecting the operation of various circuits therein. For example, an RC time constant can affect switching times of some circuits, determining the amount of time required to switch from a first state to a second state in digital circuitry. It is also affecting the frequency properties of RC filters.
Correct time constants of RC-circuits are particularly important in communication apparatuses. To reach a required frequency selectivity a wireless user equipment (UE) should have an approximately flat frequency response for the wanted signal, i.e. in-band, and a deep attenuation of unwanted signals, i.e. out-of-band. In a zero-IF (Intermediate Frequency) receiver the frequency selectivity is achieved by a variable bandwidth LPF (VBWLPF) as the desired signal is located around DC (Direct Current, i.e. zero frequency).
For 2G/3G/4G capable UEs, the LPF cut-off frequency needs to be configurable over a wide range of frequencies. For instance, the channel bandwidth can range from a few 100 kHz, e.g. GSM, to 36 MHz, e.g. 2×LTE20, with more than 10 different operation modes.
The accuracy of the LPF cut-off frequency is also important. If it is too wide, the unwanted interference may compress the filter, and if it is too narrow the in-band signal may be filtered out. In both cases, the signal-to-noise ratio (SNR) for the desired signal is degraded. The pole locations of an integrated active-RC LPF is determined by the RC time constant of on chip resistors and capacitors. Due to e.g. manufacturing inaccuracies and temperature variations, the cut-off frequency might deviate by as much as 40% from its nominal (“designed for”) value in practice. To achieve an accurate cut-off frequency it is common practice to use an RC calibration circuit. It generates a digital control signal, used to adjust digitally tunable resistors and/or capacitors of the LPF to achieve the desired pole locations.
One method for finding the RC time constant in a circuit, e.g. on chip in an integrated circuit, is to adjust the RC time constant of an RC-oscillator until its oscillation frequency equals that of an accurate frequency reference. An example of such a technique is disclosed in US 2013/0082720A1, which describes RC calibration circuit suitable for oscillators. Using chopping, the offset effect of the comparator and current source mismatches can be compensated for. However the switching time for the discharge period is ignored, leading to a measurement error. Also the calibration method may need many cycles to get a result. Note that the oscillator based calibration needs a long time to get results, and the results are also temperature sensitive as they depend on the time it takes for switching devices to completely charge/discharge capacitors, and the speeds of switching devices are temperature dependent.
Another approach is to instead use the kind of calibration circuit shown in FIG. 1. It also locks the RC time constant to an accurate frequency reference, but does not require an oscillator. Instead, it has a comparator and a pair of current sources, one connected to a digitally tunable resistance R and the other one to an equivalent dynamic resistance Req created by switched capacitors, C. The (accurate) switching frequency is here denoted fsw providing the equivalent dynamic resistance Req as fsw·C. If R is adjusted such that voltages across the digitally tunable resistance and the equivalent dynamic resistance are equal, the time constant is given by RC=1/fsw. The correct setting of R is found by first setting it to its smallest value, by setting a digital control value to a smallest value, and then increasing the digital control value, and thereby R, step by step until the comparator output, DATA_READ, toggles. If low-pass filter (LPF) resistors are of the same type as the resistors of the calibration circuit, the digital control value for R can be used directly in the LPF without the need for any further processing. A filter built with three capacitors, here 2 pF, and two resistors, here 20 kΩ, is preferably connected at the comparator input to improve the accuracy of the circuit by reducing the switching noise from the switched capacitors, and reducing kickback effects from the comparator. For the comparator input voltage to settle, the updating of R must be made at a lot slower pace than the switching frequency. The switching noise of the switched capacitors is further filtered by a relatively large capacitor, Cbig. To make the current sources well matched, and reduce the offset of the comparator, relatively large transistors are required. To keep the thermal noise low, R must be small, or the time constant for the filter at the comparator input must be large. Also note that the small switch resistance means large switching devices, which contradicts the requirement for small parasitic capacitance meaning small switching devices. Due to high time constants and large switching devices the calibration time is rather long, about 10 μs.
US 2009/0140701A1, which relates to Auto-averaging RC time constant calibration, shows an RC time constant calibration circuit using two comparators and two switched capacitors to do the RC time constant measurement. The reason for using two capacitors/comparators is to remove the effect of the discharging period as the capacitors are charged in an interleaved way. No offset effect of the comparator is taken into account.
Thus, some of the present oscillator-based RC calibration solutions are sensitive to temperature and requires a long switching time. Other solutions for RC calibration may show other drawbacks, such as:                Accuracy. As the mismatch in the current sources and comparators lead to a certain error that is not negligible, the accuracy is limited. The digitally tuned resistor should preferably have a relatively high resistance to achieve a relatively low current consumption. However, this leads to relatively high voltage noise.        Speed. The digital control is created by a test procedure that needs to change settings for a variable capacitor or resistor until voltages across the digitally tunable resistance and the equivalent dynamic resistance are equal. It means that every setting must be tested in the worst case. Due to the analog filters for the comparator input, the settling time is long making the circuit slow.        Cost. Analog circuit elements like the relatively large capacitor, Cbig, the filter at the comparator input, well matched current sources, and a low offset comparator occupies significant chip area since the matching is better for components with large area.        
It is therefore a desire to provide an alternative way of RC calibration measurements.